Upgrading boss gear drops

[sox_n_sandals]

So me and my friend have been playing the game a lot and across our runs we've collected a healthy amount of boss drops. The thing that bugs me is the fact all of the boss gear has a viewable upgraded state that i cant seem to figure out how to reach. To upgrade an item you need a duplicate of it and since they're unique boss drops it doesn't seem like that's an option. if anyone could give some answers on how to upgrade the boss items that would be really helpful. i cant imagine the developers would implement an upgraded state for them with no way to reach it

[Fendelphi]

There are some non-unique boss drops that a boss might drop, which can be "easy" to upgrade through dublicates from various shops.

Of the unique drops, the only way I know of, is to play on a high Madness level. This increases the chance of any reward to be "corrupted"(purple), including boss drops and event items.
Basically, it is up to RNG.

[[byah] WingZeroCustom]

For unique equipment in the "Weapon" slot, the Rat King in the Sewers in the ice level can upgrade a weapon at the cost of that character's jewelry and 12 max HP reduction.

[Shneekey]

Originally posted by byah WingZeroCustom:

For unique equipment in the "Weapon" slot, the Rat King in the Sewers in the ice level can upgrade a weapon at the cost of that character's jewelry and 12 max HP reduction.

There's also a way to bypass the jewlery slot sacrifice depending on the route you take in the sewer and the interaction you have.

[Urgash]

Originally posted by Shneekey:

Originally posted by byah WingZeroCustom:

For unique equipment in the "Weapon" slot, the Rat King in the Sewers in the ice level can upgrade a weapon at the cost of that character's jewelry and 12 max HP reduction.

There's also a way to bypass the jewlery slot sacrifice depending on the route you take in the sewer and the interaction you have.

I've came across a way to avoid giving my life away, but the rat king took my jewelry.
so both options must exist.

[Syncourt]

I was wondering if items you had equipped had an increased chance to show up in stores.
If they don't, they really should.

[Falcon]

At higher and higher madness levels, equipment can become cursed (the upgraded version). So if you defeat them on a madness level they have a chance to drop there upgraded, that chance goes up the higher madness level u go